FIG. 1 illustrates a simplified block diagram of an equipment configuration for one terminal of a communication link which includes a near end hybrid. The communication link has a near-end 8 comprising a telephone 2, a four-to-two wire hybrid circuit 3, an echo canceller circuit 4, a filter 5 and a NLP 6. A far-end connected to communication network 23, can be similarly configured but is not illustrated in FIG. 1. During a conversation between the near-end and far-end users, the far end signal, x, which contains both the far-end user's speech and incidental background noise, enters the near-end 8 as signal x at node 9.
FIG. 2 illustrates a representative communication link 20 between two telephones 24 and 25. The link is comprised of a near-end 21, a far-end 22, and a communication network 23 that interconnects the near-end 21 and far-end 22. The near-end 21 has a user telephone 24, a hybrid circuit 26, and an echo canceller circuit 28. Similarly, the far-end 22 has a user telephone 25, a hybrid circuit 27, and an echo canceller circuit 29. Far-end signal power, X, is received by the near-end. Signal Y is the coupled echo signal from the far-end signal as well as the near-end signal produced by telephone 24. This near-end signal contains both the speech of the near-end telephone user and the background noise of the user's environment. Together, the near-end signal and far-end echo signal are represented by Y.
The far-end signal is provided to the four-to-two wire hybrid circuit 3 and then to near-end telephone 2. Due to the unavoidable non-linearities present in the hybrid circuit 3, some portion of the far-end signal power is coupled onto the output 7 of the hybrid circuit 3 as an echo. A composite signal y exists at node 7 containing the echo signal and the combined speech of the near-end user and any incidental background noise from the near-end user's environment. A filter having a filter length period selected and designed to be longer than the hybrid dispersion time is used prior to power level measurements at 7 to allow the echo canceller 4 to operate properly.
Echo canceller 4 synthesizes the expected value e of the echo signal in adaptive filter 5, and subtracts this value at 10 from the composite signal y existing at node 7. The resulting difference signal, d, existing at node 14, is intended to contain only the near-end signal s originating from telephone 2. Ultimately, difference signal, d, is provided to the far-end telephone through the communications network 23.
Methods of measuring the echo return loss typically measure a signal at node 9, where the signal power from the far-end would normally exist. A measurement of the signal power, x, at node 9 is made. Additionally, the power level of the composite signal y, comprised of the coupled echo signal and any signal s generated by the near-end telephone 2, is measured at node 7. The measurement can be made when little-to-no signal is being generated at near end telephone 2. Assuming the signal power of any signal generated by the near-end telephone is very small in comparison to the coupled echo signal power, the ratio of the measured test signal power x to the measured power level y provides an estimate of the echo return loss (ERL) for the near-end 8. The magnitude of echo return loss is usually measured as a difference in dB between signal x and signal y. As described in the co-pending application Ser. No. 10/029,669, incorporated herein in its entirety by reference, echo return loss may be measured dynamically during the course of a telephone conversation.
Echo is an important factor in communications which include a hybrid between a four wire communication network 23 and the end terminals 24 and 25 as illustrated in FIG. 2. When echo is present, it is preferable to eliminate the echo. To eliminate the echo, the magnitude of the echo must be determined. One way of determining the magnitude of the echo is through echo return loss (ERL) estimation. A high echo return loss means that there is very little echo because most of the energy from the far end has been lost when the near end signal combined with echo is measured.
A typical echo canceller, as illustrated in FIG. 1, includes an adaptive finite impulse filter FIR 5. Under the control of an adaptation algorithm, FIR filter 5 models the impulse response of the echo path. A non-linear processor (NLP) 6 can be used to remove residual echo that may remain after linear processing of the input signal. The echo canceller may also typically include a double talk detector 11. Double talk occurs when both far end and near end speech are present at the same time. A double talk detector 11 can also be used to control and inhibit the adaptation process of the FIR 5 and/or the NLP 6 when double talk is present and it may be undesirable to cancel or suppress echo because double talk will be suppressed.
In the echo canceller, the signal y is the perceived near end signal. Signal y is a combination of the actual near end signal s and the echo from the far end signal x which comes through hybrid 3. The output signal d is the signal y less the echo estimate e generated by the adaptive filter 5. The adaptive filter 5 is programmed to generate an output signal e that is as close to the echo as possible so that the echo is largely cancelled by the echo estimate e and the difference signal d closely resembles the generated near end signal s. The NLP 6 controls the amount of signal d that is transmitted to the far end. When there is no near end signal s, or a large echo over riding near end signal is present, NLP 6 can provide comfort noise to the far end instead of near end signal so as to prevent any possible uncancelled echo from being transmitted. When a valid s exists, NLP opens so as to let the far end hear the signal. False detection of a lack of near end signal s can cause clipping of speech and failure to detect echo can result in echo leak through the NLP. The NLP as an on/off switch can result in abrupt audible changes which are undesirable in speech communications.